(CBS News) -- Mitt Romney may be the Republican Party's presumptive nominee, but it will be a while before he actually has the delegates needed to clinch the nomination.
Romney currently has 645 delegates, according to CBS News estimates. He now needs 499 more to reach 1,144 - the number of delegates a candidate must have to be nominated.
Even though Romney's chief rival, Rick Santorum, has suspended his campaign, states continue to hold primaries. There are five contests on April 24th with 219 delegates at stake; but even if Romney won every single delegate that day he would be far short of 1,144. Wyoming and Missouri are holding conventions before April 24th where Romney may pick up some delegates but even winning all of them wouldn't bring him much closer to the magic number.
Looking ahead, Romney would need to win about 80 percent of the delegates available in the April and May events to possibly clinch on May 29th when Texas holds its primary with 152 delegates at stake. If Romney won 90 percent of the delegates in April and through May 22nd he may still come up short of the 1,144 and need delegates from Texas' primary on May 29th.
There are a couple ways Romney can get more delegate support aside from the remaining primaries: Unpledged delegates and upcoming state conventions.
There are about 120 delegates who are members of the Republican National Committee and are technically unpledged and free to support who they choose (similar to superdelegates on the Democratic side). According to CBS News estimates, Romney currently has the support of 34 of these delegates (these are included in the 645 total for Romney), but most of these delegates are uncommitted at this point. With Santorum now out of the race many of these delegates may be more likely to express their support for Romney.
Also, many of the states that held caucuses earlier in the primary season will be holding conventions soon (many in early May), which is often where the delegates are actually chosen. These delegates sometime reflect the results of the caucus results but sometimes they don't. Romney could get a few more delegates from these conventions.
As for Santorum's 252 delegates, he basically gets to keep them for now. According to the Republican National Committee, since Santorum suspended his campaign and didn't actually end it, this becomes a state by state issue. In states that bind delegates for a specified number of ballots, the delegates remain bound until they are released by the campaign. In other states that do not specify, they are free to go where they want as soon as someone's campaign is suspended.