Secret Service chief Kimberly Cheatle was shielded by her own officers after furious US senators pursued her down the corridors of the RNC, demanding she explain how a gunman was able to get a clear shot at Donald Trump.

Cheatle was in the Milwaukee convention hall to oversee security arrangements on Wednesday evening -  just hours after making a 'cover-your-a** call' to senators about the shooting in Pennsylvania on Saturday night.

Senators John Barrasso of Wyoming and Marsha Blackburn of Tennessee led the chase, complaining that they had not had a chance to put their questions to Cheatle during the call. 

A frustrated Barrasso then issued an ultimatum to the security chief.

'You put him within less than an inch of his life,' Barrasso yelled at Cheatle.  'So resignation or full explanation.'