Peter DeFazio is thought by many to be the strongest potential challenger to Smith.  In 2006 he won every county in his district, including fairly conservative rural areas.  He is a fairly populist Democrat, and it is thought that he could appeal to people from outside of the Portland-Eugene axis better than most of the other potential contenders.

In 2002, Smith faced Bill Bradbury, our excellent Democratic Secretary of State, who was not the strongest candidate.  Smith beat him in a very good year for Republicans by 712,287 to 501,898, of 1,267,221 votes cast, or 56.2% to 39.6%.

In the 1996 special election that saw Ron Wyden (D) beat Gordon Smith to replace Bob Packwood, of 1,181,650 votes cast, Wyden beat Smith 571,739, or 48.4%, to 553,519, or 46.8%.  In the 1996 general election, Gordon Smith beat Tom Bruggere (D) in the election to replace the retiring Mark Hatfield.  Smith received 677,336 and Bruggere 624,370 of the 1,360,234 votes, or 49.8% to 45.9%.  That was the last time that Smith ran during a Presidential year, which, IIRC, was a good year for Demcorats.

Here's a county map of Oregon:
http://county-map.digital-topo-maps.com/ oregon.shtml
Here you can check out Congressional District maps:
http://nationalatlas.gov/printable/congr ess.html#or

Here's a primer for Oregon:
In Oregon, Democrats hold all six statewide elected positions in the state government, and all have been held by Democrats for many years.  We now narrowly (31-29) hold the state house, and we hold the state senate (19-11).  We have 4 out of 5 Congressional districts, all but the 2nd CD.  One of our senators is Ron Wyden, a Democrat from Portland, and our other senator is Gordon Smith, a Republican from Pendleton (Eastern Oregon).

Democrats depend on Multnomah and Lane counties for strong support, because that is where Portland and Eugene are, repsectively.  Traditionally, Benton, Clatsop, Columbia, Coos, Hood River, Lane, Lincoln, Morrow, Multnomah, and Wasco County have had Democratic registration advantages, though it is often difficult to elect Democrats outside of Lane and Multnomah counties.  Clackamas County (my home county), is a swing county, with a republican advantage, and rural Clackamas and Marion counties are part of the bible belt of western Oregon.  Marion County and Washington County have had Republican advantages, with Washington County as a traditional Republican stronghold, but Democrats are now taking over the Washington County in state and national elections.  Most counties in the 1st and 3rd CDs are winnable, some in the 4th and 5th, and a couple in the 2nd, but most of the rest is deep red.  However, in 1998 Gov. Kitzhaber won all but one county, and lost that by but a few hundred votes.  Ron Wyden crushed his last opponent, too.

Significantly, when Wyden beat Smith, he won Benton County (by 2,300), Clackamas County (by 3,000), Clatsop County (by 1,500), Columbia County (by 1,001), Lincoln County (by 1,800), Lane County (by 13,300), Multnomah County (by 80,600), Tillamook County (by 600), and Washington County (by 1,100).  Smith won Coos County (by 3,100), Hood River County (by 56), Marion County (by 5,300), Polk County (by 1,100), Wasco County (by 167), and Yamhill County (by 2,900).

When Smith beat Tom Bruggere in the general election of the same year (1996), Bruggere won Benton County (by 2,400), Lincoln County (by 1,100), Lane County (by 22,400), and Multnomah County (by 73,600).  Smith won Clackamas County (by 8,000), Clatsop County (by 600), Columbia County (by 400), Coos County (by 2,600), Hood River County (by 500), Marion County (by 15,300), Polk County (by 4,400), Tillamook County (by 450), Wasco County (by 1,000), Washington County (by 8,800), and Yamhill County (by 5,600).

In 2002, when Bill Bradbury faced Smith, Bradbury won only Multnomah County (by 42,850).  It was a tough year.  Smith won Benton County (by 350), Clackamas County (by 32,000), Clatsop County (by 2,100), Columbia County (by 3,400), Coos County (by 5,250), Hood River County (by 1,050), Lane County (by 1,800), Lincoln County (by 1,700), Marion County (by 27,300) Polk County (by 7,100), Tillamook County (by 2,100), Wasco County (by 2,700), Washington County (by 32,200), and Yamhill County (by 10,150).

Conversely, in 2004, Ron Wyden demolished Republican challenger Al King.  Wyden won Benton County (by 14,400), Clackamas County (by 43,200), Clatsop County (by 6,900), Columbia County (by 8,400), Coos County (by 5,200), Hood River County (by 3,650), Lane County (by 62,950), Lincoln COunty (by 6,900), Marion County (by 25,850), Multnomah County (by 208,900), Polk County (by 6,000), Tillamook County (by 4,150), Wasco County (by 4,150), Washington County (by 72,000), and Yamhill County (by 6,800), and several other counties that rarely go blue, and severely narrowed some deep red counties.

In 2004, Peter DeFazio faced Jim Feldkamp for the 4th district, and Defazio won every county except Douglas, which he lost by 179 votes.  His district has all or part of Benton, Coos, Curry, Douglas, Josephine, Lane, and Linn counties.  In 2006, DeFazio won them all, including Douglas, which he won by 692 votes.  He swept the counties in 1996, 1998, 2000, and 2002, as well (previous records aren't available at the SOS website).  This is no easy task.  I've been told that the 4th is the most difficult district that any of our (Oregon) Democrats has, though we have a deep bench there (although I think my home district, the 5th, could be as tough).
DeFazio has a proven record of winning in base territory, in swing areas, and pretty red ones, too.  He bridges the urban-rural divide.  Short of former governor Kitzhaber, who has repeatedly made clear he has no intention of getting back into electoral politics, Peter appears to be the best candidate to rile up our base in the Portland and Eugene areas, step into those swing areas in places like Coos, Polk, and Clackamas counties, and even the deep red areas like Douglas and Jo counties, and really take it to Gordon Smith.  If we can get DeFazio to run, we will have a serious race on our hands.
With high turnout in a presidential year, the numbers show that across the state, we can take Smith down, if we have a strong candidate that can appeal outside of the Portland-Eugene axis.  We need a candidate that can pull votes out of the small towns and rural areas, not just the big, liberal cities (and Bend and Salem, two of the biggest cities, are not very liberal).
2008 should not be so rough a year as 2002, in fact, it may be almost as good as 2006, if we are not making any progress in Iraq (which, if we are still there, we won't be).  As well, Oregon Democrats are seeing red right now.  Beating Gordon Smith would be the most likely and most pleasing accomplishment that many of us could desire.  With a strong candidate, the money will be there, unlike in 2002.  But 2008 won't be like 2002 at all.  In 2008, we can do it.

All election results were derived from the Oregon SOS website: http://www.sos.state.or.us/elections/