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WASHINGTON – Labor Day is now a memory, and attention is fast shifting to the midterm battle shaping up for control of Congress.

In the Senate, where Republicans have a chance at seizing control, which incumbents are the most vulnerable?

Montana’s appointed Democratic Sen. John Walsh was by far the most endangered incumbent in the Senate in early August, but his decision to not seek a full term has opened the top slot to a couple of other worthy contenders.

Sen. Mark L. Pryor, D-Ark., is still in a perilous political position, but Louisiana Sen. Mary L. Landrieu has leapfrogged him on the list to become the Senate’s most vulnerable incumbent.

The Democrat is pushing hard to eclipse 50 percent on Nov. 4, the day of Louisiana’s jungle primary and possibly Landrieu’s best opportunity for re-election. She will undoubtedly get close. But if Landrieu doesn’t win a majority of the vote against a few GOP challengers, she will likely face Rep. Bill Cassidy, R-La., in a Dec. 6 runoff.

If that happens, all bets are off, and Landrieu’s viability may depend on which party prevailed on Election Day.

With the elections just two months away, Democratic incumbents overall have run strong-enough campaigns to ensure the fight for the Senate majority remains a toss-up – despite a playing field tilted heavily in the GOP’s direction.

Republicans, who need a net gain of six seats to take control of the chamber, are expected to get halfway there by picking up the open seats in Montana, South Dakota and West Virginia. The open-seat opportunities don’t stop there, but the GOP will likely need to defeat at least two sitting senators to win the majority. They have several from whom to choose.

Here are the 10 senators considered most vulnerable, nine of them Democrats:

1. Mary L. Landrieu, D-La. – No incumbent faces a more complicated path to re-election than Landrieu, thanks to the state’s unique voting process and calendar. With a challenging national climate and a strong GOP challenger in Cassidy, Republicans may finally defeat the senator who has won some tough races before.

2. Mark L. Pryor, D-Ark. – Pryor is surprising people in both parties and is in far better shape at this point than some expected a year ago. But he remains a top target of national Republicans, who believe Rep. Tom Cotton will ultimately win over the GOP voters who have supported Pryor in the past.

3. Mark P. Begich, D-Alaska – The Democrat running arguably the best campaign also happens to be in possibly the most challenging state for the party. Begich and his Democratic backers have been hammering Republican Dan Sullivan for months, long before he finally emerged with the nomination Aug. 19.

4. Kay Hagan, D-N.C. – Hagan is a top target for Republicans, but circumstances have been good to her. Republican Thom Tillis, the state House speaker, had to put his candidacy on the back burner for the last several months when the legislature’s “short session” ended up not so short. He only became a full-time candidate last week, when the House finally wrapped up.

5. Mark E. Udall, D-Colo. – Udall has a strong opponent in Rep. Cory Gardner, and Colorado is a solidly purple state that could go either way. Democrats are hoping their 2010 playbook, which got Democratic Sen. Michael F. Bennet elected in a Republican wave year, will be as successful the second time around.

6. Mitch McConnell, R-Ky. – If rankings went by polling alone, the Senate minority leader might be higher on this list. He faces a legitimate challenge in Democrat Alison Lundergan Grimes, a rock star fundraiser who makes the party optimistic. Grimes needs to top the 47 percent McConnell’s previous opponent received in 2008, but that’s not easy in a midterm cycle with an unpopular Democratic president.

7. Jeanne Shaheen, D-N.H. – Shaheen is well-liked and remains the clear favorite, but a single poll in mid-August showing a 2-point race caused an uproar of speculation. It’s likely not that close, but Republicans hope former Massachusetts Sen. Scott P. Brown will begin to close the gap after the Sept. 9 primary.

8. Jeff Merkley, D-Ore. – With a solid lead in the polls and large financial advantage, Merkley doesn’t appear to be in much danger. But Republican Monica Wehby is getting some much-needed help from the Koch brothers on the airwaves and Merkley’s poll numbers, while strong, don’t match Al Franken’s, pushing the Oregon senator up one spot on this list.

9. Al Franken, D-Minn. – Franken is a strong fundraiser in a solidly blue state. There’s a path for Republicans to potentially make this race competitive, but so far, GOP nominee Mike McFadden hasn’t made it happen.

10. Mark R. Warner, D-Va. – It would take a Republican wave election to oust Warner, a popular former governor with a massive cash advantage over his Republican opponent Ed Gillespie. He joins this list largely for lack of another option.