In a sign that the Republican presidential field is narrowing further, Ron Paul emailed his supporters to say he is on the verge of leaving the race unless he can raise millions more.

Former Pennsylvania Senator Rick Santorum's announcement on Tuesday that he was suspending his campaign removed the main obstacle to Mitt Romney securing the nomination, but Newt Gingrich and Paul -- both of whom lag far behind Romney in the delegate count -- have so far not followed Santorumt to the exits.

While Gingrich has vowed to sustain a campaign currently more than $4 million in debt all the way to the convention, Paul struck a more realistic note in an email to supporters. The longtime Texas libertarian has relied on a small but devoted following that allows him to raise bursts of cash in pushes called money bombs, and he called for another quick funding infusion.

I won't mince words, Paul said in an email to supporters Wednesday evening. If we don't reach our $2.5 million goal, I am not sure our campaign can go on.

Paul has built his campaign on an anti-establishment message, contending often that the Republican party has strayed from a small-government philosophy, and he struck a defiant note in asking for more cash.

In the coming days, the media will be expecting me to also bow down to their new prince, Paul wrote. Well, I want to give them a little surprise. Instead of a defeated candidate bowing out of the race, as we saw yesterday, let's show them a surging candidate determined to ride on to Tampa!