The Royal College of Nursing has not been told whether a representative from the Conservatives will address its members at congress, the chief executive said.
Labour leader Jeremy Corbyn and Liberal Democrat leader Tim Farron both spoke at the RCN congress in Liverpool on Monday morning.
Their speeches came a day after the RCN warned, unless the next government dropped the 1% cap on pay, it would hold a ballot on industrial action later in the year, threatening the first ever strikes by its members.
At Monday's afternoon session, chief executive and general secretary Janet Davies said there had been communications with the Conservative Party.
She said: "We have issued an invitation and there have been some communications but we don't know yet whether anyone is coming or not."
She said there had not been a refusal from the party to attend.
The congress is due to run until Wednesday.
The issue was raised as a point of order by member Linda Bailey who said the RCN was "apolitical" not "non-political" and leaders of all the three main parties should have been at the congress.
Mr Corbyn set out a promise to provide an extra £37 billion for the NHS when he spoke at the congress, while Mr Farron said Liberal Democrats would give the health service and social care a £48 billion funding boost over five years if they gained power.