Jeremy Corbyn will spend four days campaigning in Scotland this week in an attempt to revitalise Scottish Labour’s fortunes after another slump in polling.

Labour sources admit the party has failed to capitalise on a surge in support during last year’s general election, when Labour defied expectations by winning back six Westminster seats from the Scottish National party.

Corbyn’s tour will include a speech in Glasgow on Monday, an event at Edinburgh international book festival and visits to target seats in Lanarkshire, which Labour would have to regain from the SNP if it were to win another snap election.

There are concerns that the failure to consolidate last year’s gains will threaten the party’s wider strategy. Of the 64 target seats across the UK it must win to regain power at Westminster, 18 are Scottish constituencies held by the SNP.

Prof John Curtice, an elections expert at the University of Strathclyde, said: “Scotland is absolutely crucial to Corbyn’s hopes of becoming prime minister.”

After winning 27% of the Scottish vote in last year’s Westminster election, recent opinion polls have put Labour back in third place behind the Tories at about 23%. After 11 years in power at Holyrood, the SNP’s dominance of Scottish politics remains unchallenged, with the nationalists polling about 40%.

Those polls have raised concern that Corbyn’s attractiveness to former Labour voters who defected to the SNP has faded and that too few voters have heard of the Scottish Labour leader, the leftwinger Richard Leonard, or they are not impressed by him.

Senior party sources argue that Labour’s poll support is far better than the 14% it recorded in early 2017, but admit they are anxious. Leonard is regarded as too reserved and is under heavy pressure internally to do more to increase his visibility, but he has resisted requests to allow the media to do personal profiles or discuss his home life.

“We have plateaued. We understand that, but it has plateaued at a much stronger place than it was before,” said one senior party source. “We are fighting against two competing nationalisms and Scotland is still influenced by the constitutional debate of 2014. To get from where we were before is progress, but much more progress needs to be made and we understand that.”

Jeremy Corbyn with Richard Leonard, the Scottish Labour leader. Photograph: Murdo Macleod/The Guardian

Curtice said: “Labour is alive and kicking in Scotland [but] it still has a hell of a long way to go, to get back to anything like the level of support it had 10 years ago.”

Leonard said he was optimistic. “We are on an election footing for a general election potentially later this year, and I think that can’t come soon enough. In 10 seats we need about a 1% swing to take those seats off the SNP,” he said.

“I’m fairly philosophical about the fact that we’ve got three years until the next Scottish parliament elections, so by the time we get to 2021 I hope that my standing in the eyes of the people of Scotland will be raised.”

Corbyn’s centrist critics argue that the party’s UK leader is far less of an asset in Scotland than his supporters believe, and link Labour’s dip in support to his stance on leaving the EU. They accuse the Scottish party of failing to tap into the greater popular hostility to Brexit than in most other parts of the UK, and argue that the SNP leader, Nicola Sturgeon, is reflecting that antipathy far more successfully than Labour, particularly among middle-class voters.

They also believe the recent controversy over Corbyn’s stance on Israel has damaged Scottish Labour support in key constituencies.

They say the first major antisemitism row in May 2016, over remarks about Hitler and Zionism by Ken Livingstone, a Corbyn supporter, hit Labour’s vote in the Scottish elections and contributed to the party losing East Renfrewshire, home to Scotland’s largest Jewish community.

One leading Corbyn critic said the current row showed “his past is coming back to haunt him”.

Matt Kerr, a Labour councillor who came within 60 votes of winning Glasgow South West from the SNP last year, said of the current polls: “We need to be honest that there is a long way to go.”

He said, however, that Corbyn had “given us a foothold with voters”.

“The socialist message was lost in the constitutional battles and we’ve had to work hard to be heard again. As the third-placed party we don’t get the airplay that we used to, but at least we’re not acting like a rabbit in the headlights now,” he said.

“We know what we’re for, and that’s because of a solid policy platform and the influx of younger, highly motivated members.”