Video report by ITV News Reporter Rebecca Barry
Nurses are threatening a possible summer of protests amid falling staffing levels and anger at a cap on pay.
The Royal College of Nursing (RCN) warned that unless the next government drops a 1 percent pay cap - introduced in 2015 to great opposition - it will hold a ballot on industrial action later this year.
Any strike would be the first by RCN members. There is still some way to go before that, but almost four out of five nurses backed strikes in a consultative ballot, the RCN said, while nine out of 10 supported action short of a strike.
More than 50,000 of the RCN's 270,000 members took part in the poll.
Another ballot would need to be held before any action takes place.
The news comes as nursing leaders released figures showing the number of vacant nursing positions has doubled since 2013.
The RCN said that one in nine nursing posts is going unfilled and warned patients face falling care standards if safe staffing levels are not enshrined in law.
A Freedom of Information request by the nursing body showed that two-thirds of NHS hospital trusts in England planned for a greater proportion of nursing support staff last year than a year earlier.
Michael Brown, chairman of the RCN Council, said that its members "can't and won't take any more.
"This is an unprecedented show of anger and frustration over the Government's pay policy. Politicians must now listen and tell us what they will do about nursing pay.
"It's a message to all parties that the crisis in nursing recruitment must be put centre stage in this election. We're demanding answers on behalf of our patients as well as nursing staff.
"If we don't stand up now, how can we guarantee their future safety and wellbeing?"
Mr Brown continued that if a strike were to be held, it would be the first RCN members took industrial action in the group's 100 year history.
Janet Davies, the RCN's chief executive and general secretary, warned the government was open to the accusation of offering "nursing on the cheap" in place of recruiting and retaining registered and experienced nurses.
Ms Davies said: "The current conditions in the NHS are driving people out of the profession and putting new people off entering it.
"Our argument is not with patients - this is about ensuring that they get the safe and effective care they need. The 1 percent cap on nursing pay is putting patient care at risk."
Jon Skewes, director of policy at the Royal College of Midwives, said: "NHS staff have now seen seven years of pay restraint and with at least another three years on the horizon.
"Continuing pay restraint is a disastrous, unsustainable policy for maternity services and the NHS.
"We are working with other NHS trade unions to break the government's policy of pay restraint."
A spokesperson for the Conservative Party praised the work of NHS nurses.
"Our nurses do a fantastic job," the spokesperson said.
"We've had to take difficult decisions on pay across the public sector given the deficit we inherited - while continuing to boost the NHS budget so patients get high-quality care.
"We've prioritised increasing the number of nurses to help those already working hard - with 12,100 more on our wards since 2010.
"But in truth, the only way we can increase NHS funding, staffing or pay is to get a good Brexit deal so the economy prospers in the years ahead."