Is there a growing consensus on more powers for the Scottish Parliament among the parties backing a No vote in this September's referendum?

It would be hard to argue such a case convincingly.

The gulf between the visions set out by former prime minister Gordon Brown and former Liberal Democrat leader Sir Menzies Campbell remains significant.

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They are particularly far apart on the proportion of Scotland's income the Scottish Parliament should be empowered to raise: in Mr Brown's view 40%, in Mr Campbell's report up to 60%. Mr Brown's more cautious proposal is based on a view of a modernised, forward-looking United Kingdom, retaining key strengths of the Union. The figure he proposes is the maximum he believes can be offered without undermining those strengths.

Sir Menzies, meanwhile, is proposing moving towards a federal UK, with a summit to decide on greater powers for Holyrood within 30 days of a No vote. Labour's final plans will be set out at the party's conference, while the Conservatives are likely to advance their ideas in May.

Does it matter if the proposals from the main parties differ? There is a need for positive, proactive ideas from the pro-Union campaigners. With some justification, they have been accused of conducting a negative campaign.

The tactic appears to have been effective to date but undecided voters will welcome a clearer picture of what a No vote will mean, just as many seek greater clarity from Yes Scotland.

Mr Brown says Edinburgh and London alone cannot solve pressing problems relating to unemployment, housing and poverty.

A power-sharing arrangement would help tackle this, he claims. Would it? At present, the two parliaments often seem to be straining in opposite directions. This is why a clear choice between positive options is helpful.

Yes Scotland says promises of greater powers have come to nothing. Increasingly potent devolution has transformed Scotland's ability to make its own decisions. Polls suggest many favour further devolution, so it matters what that might look like.

What the two contributions leave unsaid is the degree of constitutional change such an outcome might engender across the UK. A positive No vote would probably mean a clearer vision of what Scotland gained by being in the Union. It would also probably mean a vote for constitutional change in the other countries of the Union.

Increased devolution would raise questions about extending the Welsh National Assembly's powers. There would also be a need for change in England.

Mr Brown's call for a constitutional guarantee of the permanence of the Scottish Parliament can be read as laying the groundwork for such an outcome. Mr Campbell's federal UK also presupposes as much.

There is more to be done to elaborate on these visions of a future UK. For example, a workable federal model would require careful balancing to ensure England's size did not overwhelm the other nations.

Regardless of the outcome of the referendum, voters need to feel they have voted for something constructive, not just against what they do not want. The pro-Union parties are beginning to explain what that might be but it is not yet a consensus. It might be unrealistic to expect one.

But the choice for voters from the pro-Union camps revolves increasingly around maximising the case for devolution, that work in progress.