Sen. Kyrsten Sinema of Arizona is set to be the first Democrat to work alongside Republicans on a paid parental leave idea, Sen. Bill Cassidy, R-La., said Wednesday.

"It is the first bill that is bipartisan," Cassidy said at an event about paid family leave at the conservative American Enterprise Institute.

Cassidy hasn't unveiled his legislation yet but is expected to soon. As with several other plans Republicans have rolled out in recent weeks, the idea is expected to involve the Social Security program. Other Republican plans have proposed allowing new parents to take Social Security early to pay for leave, in exchange for delaying retirement until later.

"The simple answer is: most likely," Cassidy said in a later phone call with the Washington Examiner about whether the bipartisan plan would use Social Security. "The reason I pause and hedge is because I have a bipartisan co-lead. We just established this relationship within the last week. I’m not going to presuppose where she is going to land."

Sinema confirmed on Twitter that she was working on the bill. "I’m working with @SenBillCassidy on bipartisan #paidfamilyleave because Arizona families deserve the chance to get ahead," she wrote.

I’m working with @SenBillCassidy on bipartisan #paidfamilyleave because Arizona families deserve the chance to get ahead. https://t.co/zXDQVofzWo — Kyrsten Sinema (@SenatorSinema) April 3, 2019

So far, GOP Sens. Joni Ernst of Iowa and Mike Lee of Utah have paired up for a bill, and Sens. Marco Rubio of Florida and Mitt Romney of Utah have paired up on another idea, both of which involve using Social Security.

The plans vary slightly, and Cassidy's idea is expected to have several differences as well, but he hasn't shared the details yet.

Democrats have slammed the GOP idea using Social Security as "fake leave" and have said that people shouldn't have to choose between retirement and parental leave. Most Senate Democrats have instead rallied around a plan from Sen. Kirsten Gillibrand, D-N.Y., who is also running for president, to fund paid family leave through a payroll tax.

Republicans getting behind paid parental leave is still a new development. President Trump has said in his State of the Union address that parental leave was a priority for him, and senators as well as House members have been meeting with White House adviser and first daughter Ivanka Trump to share ideas. In the House, legislation has been introduced by GOP Reps. Ann Wagner of Missouri and Dan Crenshaw of Texas. Rep. Rosa DeLauro of Connecticut has led the Democratic bill.

[Opinion: This 'Equal Pay Day,' stop advocating for paid parental leave policies that depress women's wages]