Speaking at the Conservative Political Action Conference (CPAC), Mr Romney said he was “severely conservative”.
Rivals Rick Santorum – who won a hat-trick of state caucuses and primaries on Tuesday – and Newt Gingrich also addressed the CPAC delegates.
The three are battling for the right to challenge Barack Obama in November.
Mr Romney hailed the Obama presidency as the “last gasp of liberalism’s great failure and a turning point for a new conservative era”.
Urging conservative Republicans to support his candidacy, he vowed he would be a pro-life president and would roll back regulations introduced under President Obama’s term that he said curtailed religious freedom.
He pledged to balance the federal budget and told supporters that before approving new spending, he would ask: “Is it worth borrowing from China to pay for it?”
Newt Gingrich: “America works when America is working”
The former governor of Massachusetts touted his record in office, saying he “fought against long odds in a deep blue state”.
In a speech made amid lingering concerns from conservatives about his record in office and his political convictions, Mr Romney sought to win over the crowd with frequent criticisms of the federal government and of the current president.
“I served in government, but I didn’t inhale”, he said, telling applauding crowds that he remained a businessman at heart.
Mr. Romney has long been seen as the front-runner and the favoured candidate of the Republican elite.
But he has struggled to convince conservative voters, who represent a key portion of the party’s primary-season electorate, that he is the right candidate for them.
Mr. Romney is seen as having previously held moderate views on social issues, including abortion.
‘Part of the problem’
Mr. Santorum, by contrast, has seen his long-standing support for social conservative touchstones win him new support as the primary season has unfolded.
Rick Santorum: “Our rights don’t come from the government. They come from a much higher authority”
In his speech on Friday morning he implored conservatives to stand up for their beliefs and made a veiled reference to Mr. Romney’s troubles with the Republican base.
“We’re going to win with contrasts, we’re going to win with ideas, we’re going to win by making Barack Obama and his failed policies the issue in this race.
“Why would an undecided voter vote for a candidate the party is not excited about?” he said.
Mr. Romney spent Thursday meeting potential donors and influential conservative groups in small sessions, although some skeptics of his campaign were absent, the New York Times reported.
Both Mr. Romney and Mr Gingrich assailed Rick Santorum over his record in Congress ahead of their stage time on Friday.
Mr. Romney’s campaign labelled the former Pennsylvania senator “part of the problem” on spending and debt, contending that he repeatedly voted to raise the US debt limit while in Congress, and reprised attacks on him over congressional earmarks.
Mr. Gingrich, who largely stepped aside from this week’s battles in Colorado, Minnesota and Missouri, said his conservative record was stronger than Mr Santorum’s.
“He was a big labour Republican who consistently voted with the unions” during his time in Congress, Mr Gingrich told Fox News.
Mr. Romney ended his 2008 presidential bid at that year’s CPAC event.