His nomination all but assured, Mitt Romney is set to inch closer to clinching the GOP presidential nomination with a presidential primary in Oregon.

Oregon and Nebraska are taking their turns weighing in on the Republican race, though Nebraska's contest is little more than a beauty pageant.

The major difference when Republicans go to the polls on Tuesday is the lack of opponents for Romney.

Texas Rep. Ron Paul became the last Romney rival to step aside, saying in a statement Monday that he would stop spending money to compete in primaries. He urged his fervent supporters to continue working at the state party level to cause havoc for Romney.

Just 171 delegates short of the number he needs for the nomination, Romney is the presumptive Republican challenger to President Barack Obama. He plans to spend Tuesday campaigning in Iowa, a competitive general election battleground, and delivering a speech on the economy as he looks to counter Obama on voters' top concern.

Oregon has 25 delegates at stake in Tuesday's primary. While Nebraska voters will register their preferences for a nominee, 32 delegates to the Republican National Convention will be determined at the state convention on July 14.

Nebraska voters will also select the Republican candidate to face Democrat Bob Kerrey in the state's U.S. Senate race. Kerrey served as Nebraska's governor and senator before leaving Congress in 2001 to be a university president.

Democratic Sen. Ben Nelson is not seeking another term and the Nebraska race is one both parties are eyeing as Democrats try to keep their majority in the Senate. State Attorney General Jon Bruning has enjoyed the backing from establishment Republicans, while state Sen. Deb Fischer has attracted support from former Alaska Gov. Sarah Palin, Sen. Jim DeMint of South Carolina and other nationally known tea party favorites.

Elsewhere on Tuesday, voters in Idaho are set to pick nominees for state and congressional offices.

Earlier this year, Romney won Idaho's presidential caucuses when rivals Rick Santorum and Newt Gingrich remained contenders.

Romney so far has captured 973 delegates, while vanquished foes Santorum collected 264 and Gingrich has 130. Paul has 104 delegates. The nominee needs at least 1,144 delegates.

Romney is on pace to secure the nomination before the month ends.