Most of the primaries and caucuses that have taken place so far on the Republican side have not been closed. In these contests, independents and in some cases Democrats are allowed to vote for a GOP candidate. Pointing this out, a friend who supports Cruz argued that Trump is only leading in total votes received because of his advantage among non-Republican voters.
Not true. Using exit polling data for the non-closed contests and overall vote counts for the closed contests, the total votes received by registered Republicans for first and second place:
Trump -- 3,049,777
Cruz -- 2,598,055
Trump has earned 117% as many Republican votes as Cruz has. Put in another way, among Republicans who've voted for one of the top two, Trump beats Cruz, 54%-46%. Trump's lead over Cruz isn't as wide as it is among independents, but a lead it most certainly is.
Keep in mind, too, that Cruz has had several home games early in the season. After Ohio and Florida, Trump will extend that advantage substantially as it appears unlikely that Cruz, who can't get much traction east of the Mississippi (Maine, where less than 2% of the adult population voted in the Republican caucus, was decided by zeal, not quantity) will be near Trump in either of these large states.
I didn't run the numbers for Rubio and Kasich because sifting through the figures are tedious and these third-tier guys aren't even close to Trump or Cruz in votes received, Republican or otherwise.
Speaking of otherwise, total votes received in the primaries and caucuses among the four remaining GOP contenders: