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Americans go to the polls Tuesday to renew the membership of Congress, with all 435 seats in the House of Representatives and 37 seats in the Senate on the ballot. In addition, there are governors races in 37 states.

The midterm election is the biggest test yet for President Barack Obama. Democrats up and down the ballot are expected to pay the price for his declining popularity.

Five Senate races to watch

Republicans need a net gain of 10 seats to take control of the Senate. That seems an impossibly tall order, considering the number of solidly blue states holding Senate contests this year. Even so, the GOP is showing enough strength to have Democrats worried. Here are the five Senate races that will shape the next Congress:

1. Florida

Tea-Party-backed Republican Marco Rubio looked unbeatable until reports surfaced that Bill Clinton asked Democrat Kendrick Meek to drop out and endorse independent candidate Charlie Crist. Democrats are worried a Rubio win would position the Cuban-American for a spot on the 2012 presidential ticket.

2. Illinios

Mr. Obama's former Senate seat risks falling into the Republican hands of Mark Kirk. The Obamas have campaigned for Alexi Giannoulias and appeared in a TV ad for him, one of the only races in which the President is still seen as a good salesman.

3. Pennsylvania

Mr. Obama won this state by 10 percentage points in 2008. But polls show Republican Pat Toomey, considered one of the GOP's most right-wing candidates, maintaining an edge over Democrat Joe Sestak.

4. Colorado

Republican Ken Buck is a Tea Party favourite with a habit of putting his foot in his mouth. Still, he has a razor-thin edge over incumbent Democrat Michael Bennet.

5. Nevada

Senate Majority Leader Harry Reid is the juiciest target of all for Republicans. He is in the fight of his life against Tea-Party-backed GOP candidate Sharron Angle. Based on the small number of voters, this is the most expensive race in the country.

10 House races to watch

Republicans are expected to lose no more than a handful of the seats they currently hold in the House of Representatives. But they stand to win dozens of the seats now held by Democrats. Republicans need a net gain of 39 seats to win a 218-seat majority. The size of a Republican wave will be determined by how things go in some key Democratic districts.

1. Alabama 2nd District

Democratic incumbent Bobby Bright is a staunch conservative who regularly votes against his party. If he loses, it could signal just how much of dying breed Southern Democrats have become.

2. Virginia 5th District

Freshman Democrat Tom Periello unseated the Republican incumbent in 2008 by 727 votes, the smallest margin in the country. Mr.k Obama visited the mostly rural district to mobilize African-American voters.

3. Pennsylvania 11th District

Paul Kanjorski has represented the blue-collar district that encompasses Scranton since 1984. A loss would signify just how much trouble Democrats are in with white, working-class voters.

4. Ohio 16th District

Democrat John Boccieri voted against the first health-care reform bill, but supported a final version. That switch may cost him his Akron seat.

5. Massachusetts 10th District

Until Scott Brown won his Senate seat this year, the state's 12-member congressional delegation was entirely made up of Democrats. But Republicans hope they can add another one in this Cape Cod district.

6. Missouri 4th District

Incumbent Ike Skelton has been in Congress for more than three decades. But he is in the fight of his life against a candidate backed by Sarah Palin.

7. Arkansas 1st District

This seat has been in Democratic hands since the Civil War. But the retirement of incumbent Marion Berry has increased the odds the district will go Republican this time.

8. Colorado 4th District

Mr. Obama singled out freshman Betsy Markey when he thanked members from swing districts for supporting his health-care reform bill. Ms. Markey could now pay the price for the President's special attention.

9. Washington 3rd District

With the 12-year Democratic incumbent retiring, Republicans are confident they can win this seat with Jaime Herrera, a young Hispanic member of the state legislature.

10. Florida 8th District

It would be hard to pick a Democratic seat the GOP would like to win more than this one. Freshman incumbent Alan Grayson has a talent for provoking Republicans. But he could be in trouble in this suburban Orlando seat.