When members of the now-defunct Independent Democratic Conference reunited with mainstream Democrats in the New York State Senate in early April, they probably hoped that their fledgling primary challengers would fade.

In the three months since, that has not happened.

Instead, there are increasing signs that those challengers are making inroads with members of the state’s political establishment and with local leaders in the districts they hope to represent. And in many cases, endorsements are being driven by the I.D.C.’s past association with Republicans, with whom it collaborated to help that party maintain control of the State Senate for more than seven years.

The latest example will come on Thursday, when four Assembly members from central Brooklyn will endorse Zellnor Myrie, a 31-year-old Democrat who is challenging Senator Jesse Hamilton, who defected to the I.D.C. in late 2016, saying he wanted to “get results.” That decision — either pragmatic or opportunistic, depending on your worldview — now seems like it could come back to haunt former I.D.C. members like Mr. Hamilton and his peers.

“We need progressive public servants in our State Legislature,” said Assemblyman Walter T. Mosley, one of the four lawmakers endorsing Mr. Myrie, in a statement that characterized the I.D.C. as a “broken model of political expediency and self-interest.” “Zellnor Myrie is that leader.”