BROOKFIELD, Wis. (Reuters) — U.S. presidential hopeful Rick Santorum vowed on Saturday that the race for the Republican nomination is far from over but signaled that he would not be “unrealistic” if the time ever came to step aside for rival Mitt Romney.

Lagging in opinion polls, endorsements and delegates, Santorum nevertheless challenged the Romney camp's assertions that the former Massachusetts governor is the inevitable Republican nominee to face President Barack Obama on Nov. 6.

“They put on the facade of inevitability, and they realize — I realize — this is far from over,” Santorum said in an interview with Reuters in Wisconsin.

Endorsements this week from former President George H.W. Bush and Florida Sen. Marco Rubio, a favorite of the conservative Tea Party movement, have buoyed Romney ahead of Tuesday's Wisconsin Republican presidential primary.

Romney led Santorum by 7 percentage points in an NBC/Marist poll of likely Wisconsin primary voters on Friday.

Santorum could come under increasing pressure to pull out of the race if he does not win Wisconsin. The conservative former U.S. senator also faces a tough set of primaries on April 24, including a serious challenge from Romney in Santorum's home state of Pennsylvania.

Santorum said he would not be unreasonable if Romney built an unassailable lead in the race for the nomination.

“When I feel comfortable that we've done the best we possibly could and there's just no more we can do and this race is, you know, we've run the course, then you know I'm not an unrealistic person,” Santorum said.

“I mean if that happens — I don't believe it's going to happen, but if it does happen — you know then we'll face it, we'll cross that bridge. But until that point — less than half the delegates have been voted for — I mean we've got a long way to go in this race,” he said.

In a state-by-state battle that began in January, the candidates have been competing to amass the number of delegates needed to clinch the party's nomination at its convention in August.

Santorum said he was not paying too much attention to senior Republicans calling on him to step aside and let Romney focus on the fight with Obama in the general election.

“When people say ‘Oh, they want you out,' well none of these people were for me anyway,” Santorum said. “And none of these folks represent the values that I bring to the table and have been out there doing what we're doing,” he said.

“I take their comments with a grain of salt.”

Santorum, while serving in the Senate, made a campaign trip in 2004 in support of President George W. Bush during the New Hampshire primary, but Bush has shied away from endorsing anyone. Bush's father backed Romney this week.

Recalling the New Hampshire trip, Santorum smiled, adding sarcastically: “And of course the Bushes have stood right behind me.”