In Ukraine, they were known by business associates for touting their connections to the Trump administration. In the U.S., their flashy style and propensity for name-dropping raised eyebrows. Their business deals at times ended in lawsuits.

The Soviet-born Florida businessmen Lev Parnas and Igor Fruman—indicted Thursday on campaign finance violation charges that include channeling foreign money into U.S. politics—boasted on social media and to associates about their close ties to American politicians, including President Trump.

Mr. Parnas made his first-ever federal political donation two weeks before the 2016 election, giving about $100,000 to Mr. Trump and his Republican Party allies, Federal Election Commission records show. He and Mr. Fruman were the first two in a long line for a photo with the president-elect at a donor thank-you event in Florida a few weeks after he won, according to a person familiar with the event.

Since then, they have sought to expand their political and business portfolios, making about $1 million in campaign contributions in recent years as they attempted deals in the liquefied-natural-gas market and Nevada’s legalized marijuana trade, according to FEC records, interviews and the indictment.

They picked up the pace in the spring of 2018, contributing to federal and local Republican candidates and political-action committees ahead of the midterm elections. Mr. Fruman attended a donor event that March at Mr. Trump’s Mar-a-Lago resort and has been photographed with the president, according to Mr. Fruman’s social-media postings, which are included in a complaint about the donations filed by the Campaign Legal Center, a nonpartisan activist organization based in Washington.