A few facts.
There are not enough Labour target seats for them to win independently.
All parties opposing the Conservatives will have limited resources. The more seats you target the more resources you need. Resources are not just money but people on the ground too.
The local candidate can increase votes; so can a group of activists who know how to campaign. However, seats change hands dramatically, as we saw with the Red Wall seats, because of a carefully targeted campaign.
For every seat the Conservatives lose at the next GE they reduce the Conservative majority by two.
There are currently 360 Conservative MPs. In theory, with 650 MPs, 326 are needed to have a majority of 2. It isn't that many. The Speaker and Deputy Speakers don't vote (4 votes). Sinn Fein does not take up their seats in Parliament (7 votes). Therefore 320 seats will work as a majority.
If Sinn Fein maintains their 7 seats in the next election and the Conservatives lose 20 seats overall, although they would still form the government, they cannot guarantee they would win any votes in Parliament.
The Liberal Democrats have 30 target seats - seats they have a realistic chance of winning. Neither of the two seats the Lib Dems gained from the Conservatives last year will be seen as target seats. They are seen as hard to win, safe Tory seats. It is not unreasonable to think the Lib Dems, alone, could remove the Conservative majority. If they can win half of the very winnable seats the Labour would only need to win 5.
There are around 47 so-called Red Wall Seats. They will not all flip back. A small number of them have been moving to blue for a while because of demographic changes.
Suppose that the Lib Dems take 15 of their target seats. Let us also suppose Labour win back 40 Red Wall seats. This is on the pessimistic side. Let's also assume neither party gains anything elsewhere. This would mean the Conservatives were still the largest party in parliament. It would also mean they do not have a parliamentary majority even with backing from the DUP.
We can expect the Tories to say that only they can form a strong government and that Labour cannot win enough seats on their own. There will be talk of a messy hung parliament.
However, the election will take place after a decade of Conservative power, with the economy quite likely in ruins and where floating voters are unlikely to have the strong objections to the alternatives that they did in the last election. So many may think a hung parliament better than continuing with a Conservative government with very limited power in Parliament. Conservatives would not be able to legislate without cross-party support.
So what if the LDs won 20 of their winnable "target" seats and Labour won 80 around the whole country. This would put Labour on 278 seats to the Conservative 260. Labour could now form a minority government. While not ideal it would mean removing the Conservatives power in Parliament and also removing them from Government as well. They would lose all control.
This would still mean vote trading would take place. This can slow legislation; that does not please the voter. However, articles in mainstream papers are talking about relationships being built ahead of the GE. They are talking to each other and discussing common areas of interest. This is bound to include electoral reform which is a top policy aim for many who back either the Labour or Lib Dem parties and smaller parties who may not be part of the discussions. This is as much in Labour's interest as it is in the Lib Dems and all other parties, members and supporters other than the Conservative Party.
This sounds like a Progressive Alliance (not an electoral alliance), i.e., joining forces for a legislative programme once the balance of power has been decided. It can be used where no party has enough seats to drive an agenda that is against the will of all the other MPs. It is being used by the SNP and Greens in Scotland and Labour and Plaid in Wales.
This is tough for Labour and the LDs but it is possible. It seems very possible they the Conservatives will lose parliamentary power but, as things stand in the poll ratings, out of the government as well.
Sadly, this is also the reason why the Dirty Parliament may keep Johnson in power. They believe their biggest threat is that there is no one else in their party who "may" be able to hang on to Red Wall seats.
Source material: A Different Bias