Former Massachusetts Gov. Mitt Romney has quietly launched a 15-city push to secure financial commitments from big-money "bundlers," hoping to reveal a fund-raising network that would establish him as the prohibitive frontrunner for the Republican nomination for president.

Mr. Romney and top aides will meet Thursday in New York with nearly 100 donors—many from Wall Street— at the Harvard Club. Attendees are being asked to raise between $25,000 and $50,000 for Mr. Romney within 90 days, in an effort to post large fund-raising...