TAMPA, Fla. - On Thursday night, Mitt Romney will formally accept the Republican presidential nomination here, kicking off the fall campaign. But if he's elected, his agenda will be driven by the outcome of U.S. Senate elections.

For all Republicans, the main focus this fall will be on electing Romney, handing him a Senate majority and maintaining the GOP majority in the House of Representatives. But for supporters of limited government, merely gaining the three seats necessary for Republicans to win control of the Senate (with Paul Ryan as the tie-breaking 51st vote) is not sufficient.

Many conservatives and libertarian-leaning Republicans remember all too well what happened about a decade ago. Even though Republicans controlled the White House and both chambers of Congress, President George W. Bush drove the agenda and spending soared. Most egregiously, through a combination of arm-twisting and skullduggery, Bush was able to muscle the largest expansion of entitlements since the Great Society through the GOP-controlled House -- the Medicare prescription drug law.

Though Romney escaped the Republican primaries, many conservatives have been uneasy about the former governor of Massachusetts, whose signature legislative achievement was a big-government health care law that was the model for Obamacare. Tea Party-style groups were active in Republican Senate primaries early on, in hopes of reshaping the chamber to help ensure a President Romney would pursue a limited-government agenda.

"When Bush took office, the House and the Senate waited for Bush to tell them what to do," explained Matt Kibbe, president of the grassroots activist group FreedomWorks. "I don't think that's going to happen next year. I think you're going to see the Senate drive the process."

Kibbe said FreedomWorks expects to spend $15 million to $20 million this election cycle through its super-PAC, with most of those funds being spent on Senate races.

The ultimate goal, Kibbe said, is to make sure that what he refers to as the "freedom caucus" of principled limited-government lawmakers becomes a majority within the broader Republican Senate caucus. If the center of gravity shifts toward the limited-government wing, Kibbe suggested, then Senate GOP leader Mitch McConnell and other deal-making Republicans would have to move in that direction to get anything accomplished. The "freedom caucus" swelled in 2010 with the election of several new senators, including Ron Johnson in Wisconsin, Marco Rubio in Florida, Rand Paul in Kentucky, Pat Toomey in Pennsylvania and Mike Lee in Utah.

Kibbe thinks they are now about three senators short of their goal, an estimate based on the fact that this was the narrow margin by which establishment-backed Sen. Roy Blunt of Missouri beat out the Tea Party-backed Johnson in an internal Republican Senate leadership fight last December.

Several Tea Party-backed Republican Senate candidates whom I spoke with this week echoed Kibbe's message.

"If we have a critical mass of strong free-market conservatives in the Senate," Ted Cruz told me Wednesday, "then I believe we will be able to drive a serious constitutional agenda to tackle [our nation's] grave fiscal and economic challenges." Cruz recently pulled off a come-from-behind victory in the Texas U.S. Senate primary. Tuesday evening, he was one of the featured speakers at the convention.

On Sunday, Ohio Republican U.S. Senate candidate Josh Mandel told me that lessons he learned by hearing the stories of his Holocaust survivor grandfather and by serving in the Marines made him confident he had the "backbone" to stand up to the inevitable pressure from party leaders to set aside his small-government principles for the good of the party.

"The day that some Republican Party leader puts his hand on my shoulders and says, 'Listen, son, you better vote for this legislation or else I'm going to kick you off of your committee, or else I'll shut off your fundraising,' I'll look that Republican boss in the eye and tell him I don't work for him and he can't push me around, because I've been through tougher stuff than this," Mandel vowed.

Whatever Romney says in his acceptance speech, the direction of his presidency -- should he win -- will likely be determined by the outcome of about a dozen U.S. Senate races.

Philip Klein (pklein@washingtonexaminer.com) is a senior editorial writer for The Washington Examiner. Follow him on Twitter at @philipaklein.