But after failing to seize control of the Republican nomination race on "Super Tuesday," Romney cannot shake off rival Rick Santorum, whom he criticized on Thursday as a political insider.
Opening up a new front against Santorum, the Romney campaign accused him of being a lobbyist in his home state of Pennsylvania even before he went to Washington in 1991.
Romney's campaign also attacked Santorum for holding regular meetings with lobbyists as a Senate leader, part of the Republican "K Street Project" seeking to increase the party's influence in the capital.
"Senator Santorum's claims to be a Washington outsider are at odds with the facts. After serving as a lobbyist in Pennsylvania before running for Congress, Santorum became a go-to guy for D.C. lobbyists while he served in the Senate," Romney spokeswoman Andrea Saul said in a statement headlined: "Lobbyist/Congressman/Senator Rick Santorum: The Insider's Insider."
With an appeal to evangelicals and conservatives that Romney often lacks, Santorum is challenging the idea his rival is the inevitable Republican nominee to face Obama in November's election.
Santorum could pick up support in a series of primary fights this month in the kind of strongly Republican states where the former Massachusetts governor has had little success so far.
To help fend off the challenge, the Super PAC outside spending group that supports Romney - by far the biggest fundraiser in the Republican field - has been pouring millions of dollars into anti-Santorum advertising.
Conservative Kansas, where Santorum is the favorite, holds its caucuses on Saturday. Alabama and Mississippi, two Southern states where evangelical Christians are a big voting bloc, have primaries on Tuesday.
Romney would dearly like to win in any of those strongly Republican states, after securing most of his primary and caucus victories to date in states that supported Obama in the 2008 election. On Thursday, Romney picked up the endorsement of Mississippi Governor Phil Bryant.
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Restore our Future, the Super PAC supporting Romney, has been on the air in Alabama and Mississippi with anti-Santorum ads for days, spending nearly $3 million.
The group is already buying $1.4 million in advertisements in two states that do not vote for weeks. Restore our Future spent more than $909,000 in Illinois, which holds its primary on March 20, and more than $431,000 in Louisiana, where the primary is March 24.
A pro-Santorum Super PAC, called the Red, White and Blue Fund, retaliated by spending $500,000 to air ads attacking Romney in Alabama and Mississippi.
Santorum spent Thursday campaigning in Alabama, where he called for a strong defense at a state-owned space museum.
Romney campaigned in Mississippi, where during a stop in Pascagoula, he said longtime aide Garrett Jackson, a graduate of the University of Mississippi, had been instructing him on the ways of the South. "I'm learning to say ya'll, I like grits," Romney said.
Also campaigning in Mississippi was Newt Gingrich, a former speaker of the House of Representatives from Georgia.
Now running third in national polls, Gingrich canceled campaign stops in Kansas to stake his campaign on strong performances in Southern contests near his home state. He has refused to heed calls to leave the nomination race.
Romney won six of the 10 states up for grabs in this week's "Super Tuesday" races, but Santorum won three and Gingrich captured one, keeping alive both their hopes and the likelihood the nomination race will continue for months.
(Additional reporting by Alina Selyukh, Alexander Cohen and Steve Holland; Editing by Peter Cooney)
