Republican Jeb Bush's new campaign ad ridiculing Donald Trump as a "germophobe" and Democrat-in-disguise is just the start of what will be a bigger onslaught, signaling a more aggressive stance by the candidate, Bush advisers said on Wednesday.

Some confidants of the former Florida governor have been seething for weeks at Trump's taunts and attacks on Bush. They have been urging the campaign to shift from the cautious approach that has distinguished Bush's early months as a candidate.

The two men are among 17 candidates for the Republican nomination in the November 2016 presidential election.