t.w._shannon.jpgOKLAHOMA CITY (AP) _ With both of Oklahoma's U.S. Senate seats on the ballot for the first time in recent history, incumbent Jim Inhofe is seeking to fend off challengers in the Republican primary for one of the seats while two of the party's ascending stars battle for the other.

Inhofe is up for re-election to a full six-year term, but fellow GOP U.S. Sen. Tom Coburn's seat will be on Tuesday's ballot too. Coburn sent shock waves through the state's political scene when he announced in January that he was stepping down due to a recurrence of cancer.

The race to replace Coburn drew three Democrats and seven Republicans. Oklahoma has not elected a Democrat to an open U.S. Senate seat since David Boren in 1978, and Republicans will be heavily favored to maintain it.

Two-term Congressman James Lankford, 46, and 36-year-old T.W. Shannon, Oklahoma's youngest and first African-American speaker of the House, both aimed to win the GOP nomination outright. But a runoff was a strong possibility with five other candidates on the GOP ballot, including state Sen. Randy Brogdon, a 61-year-old tea party favorite who ran against Gov. Mary Fallin in 2010.

Lankford and Shannon have emerged as the top contenders, with both far outpacing their opponents in fundraising. Lankford reported total contributions of about $2.1 million, while Shannon reported raising about $1.4 million.

Both candidates also have enjoyed support from “dark money'' groups that aren't required to list their donors. Much of that support came in the form of attack ads that drew a swift rebuke from Coburn, a political maverick who remains wildly popular in Oklahoma.

Lankford has campaigned as a workhorse and has risen quickly during his three years in Washington _ earning coveted assignments on both budget and oversight committees, as well as becoming chairman of the House Republican Policy Committee.

But Shannon, who also is a member of the powerful Chickasaw Nation, describes himself as the more conservative candidate and has aimed to exploit the sliver of room to Lankford's right, attacking the congressman for his votes to increase the debt limit and to support a bipartisan budget agreement.

In the other U.S. Senate race, 79-year-old Inhofe faces four little-known challengers in the GOP primary. There is no Democratic primary in that race.

In other action on the ballot Tuesday, Gov. Mary Fallin faces a pair of pro-marijuana Republican challengers in her bid for a second term as the state's chief executive, while two Republicans and four Democrats are looking to oust Superintendent of Public Instruction Janet Barresi.

Republican voters will decide winner-take-all primary elections in the race for corporation commissioner and insurance commissioner, while voters from all but the 1st Congressional District in Tulsa will head to the polls to determine their party's nominee to serve another two years in Congress.