Cairns and eight former Welsh secretaries back Remain
Welsh Secretary Alun Cairns has joined eight of his predecessors in backing a Remain EU referendum vote.
The nine politicians have made their appeal to the public in a joint letter.
But Leave campaigners including ex-Welsh secretaries David Jones and Cheryl Gillian have not lent their support.
Former Prime Minister Gordon Brown warned many Labour supporters may stay at home and not vote.
As well as Mr Cairns and Mr Crabb, signatories to the open letter included former Conservative Welsh secretaries Lord Crickhowell, Lord Hunt and Lord Hague.
Labour Welsh secretaries Lord Hain, Lord Murphy, Alun Michael and Ron Davies also supported the letter, which said: "It's not an exaggeration to say that Welsh businesses, Welsh farmers and Welsh universities could suffer enormously if we left the single market."
Conservative MP David Jones, who was Welsh Secretary between 2012 and 2014 and is leader of the Vote Leave Cymru campaign, was not a signatory.
At a campaign event in north Wales on Friday, Mr Jones said: "Sovereignty is the key issue of this referendum.
"The question is whether we govern ourselves, whether we make our own laws, whether our parliament is sovereign and whether we want to rely upon ourselves and have the confidence in our country, and whether we want to rely upon laws that are initiated by unelected bureaucrats in Brussels.
"Now is our only opportunity, our once in a lifetime opportunity, to make Britain an independent country again."
Former Prime Minister Gordon Brown spoke at the Hay Festival on Saturday, warning that recent polls suggested "a large number of Labour supporters may not support Remain but instead remain at home".
"In one recent poll as many as 62% of skilled workers are at risk of voting Leave unless we send out positive messages on employment rights," he said.
"They feel economically insecure, they don't like the status quo and they need to know that Europe offers something better for their future."
After hosting a Q&A session at the Hay Festival, he told BBC Wales a "positive, principled, progressive message" on European membership was needed to help the remain camp to convince voters.
Meanwhile, Plaid Cymru leader and Remain campaigner Leanne Wood urged voters not to see the EU referendum as an opportunity to land a blow on Prime Minister David Cameron.
"This isn't and shouldn't be about deciding which Tories we side with. It should be about our future and the futures of our childrens' generation," she said.
"This issue is far bigger than the Tory party or people's feelings towards them - this decision on June 23rd will not only shape our future but our children's too."
The leader of UKIP in the assembly, Neil Hamilton, accused Ms Wood of "patronising words", saying she "seems to think the electorate are a simple folk who will not consider the arguments properly before casting their vote".
A number of campaign events are taking place across Wales on Saturday on both sides - with Mr Crabb and Mr Cairns visiting Swansea Bay University Campus and Vote Leave Cymru campaigning in Cowbridge.
Vote Leave Cymru's Ross England said immigration would put pressure on the availability of housing.
He said: "For as long as we remain in the EU, the UK government will be completely unable to control its own borders, placing huge pressure not just on public services but on the availability of housing."
Britain Stronger In Europe has argued that a Leave vote would "destroy" young people's hopes of getting on the housing ladder.