Measurable effects of foreign election and domestic interference in elections are zero.

From The Minimal Persuasive Effects of Campaign Contact in General Elections: Evidence from 49 Field Experiments by Joshua Kalla and David E. Broockman. From the Abstract:

Significant theories of democratic accountability hinge on how political campaigns affect Americans’ candidate choices. We argue that the best estimate of the effects of campaign contact and advertising on Americans’ candidates choices in general elections is zero. First, a systematic meta-analysis of 40 field experiments estimates an average effect of zero in general elections. Second, we present nine original field experiments that increase the statistical evidence in the literature about the persuasive effects of personal contact 10-fold. These experiments’ average effect is also zero. In both existing and our original experiments, persuasive effects only appear to emerge in two rare circumstances. First, when candidates take unusually unpopular positions and campaigns invest unusually heavily in identifying persuadable voters. Second, when campaigns contact voters long before election day and measure effects immediately — although this early persuasion decays. These findings contribute to ongoing debates about how political elites influence citizens’ judgments.

Interesting. Broadly consistent with most the research I have seen, including that related to product specific advertising.

Even the New York Times acknowledges the absence of effect, citing this and other research. From Fake News Sways Elections, Right? Sorry, That’s Fake News by Brendan Nyhan.

How easy is it to change people’s votes in an election?

The answer, a growing number of studies conclude, is that most forms of political persuasion seem to have little effect at all.

This conclusion may sound jarring at a time when people are concerned about the effects of the false news articles that flooded Facebook and other online outlets during the 2016 election. Observers speculated that these so-called fake news articles swung the election to Donald J. Trump. Similar suggestions of large persuasion effects, supposedly pushing Mr. Trump to victory, have been made about online advertising from the firm Cambridge Analytica and content promoted by Russian bots.

Much more remains to be learned about the effects of these types of online activities, but people should not assume they had huge effects. Previous studies have found, for instance, that the effects of even television advertising (arguably a higher-impact medium) are very small. According to one credible estimate, the net effect of exposure to an additional ad shifts the partisan vote of approximately two people out of 10,000.

In fact, a recent meta-analysis of numerous different forms of campaign persuasion, including in-person canvassing and mail, finds that their average effect in general elections is zero.

Field experiments testing the effects of online ads on political candidates and issues have also found null effects. We shouldn’t be surprised — it’s hard to change people’s minds! Their votes are shaped by fundamental factors like which party they typically support and how they view the state of the economy. “Fake news” and bots are likely to have vastly smaller effects, especially given how polarized our politics have become.

That is all consistent with much of the earlier findings from the Mueller investigation - the amount of Russian ad purchasing and placement varied from minuscule to negligible and the impact on the election was zero.

It was disappointing to hear some senior intelligence person saying yesterday that they anticipate that the Russians were anticipated to once again successfully interfere in our 2018 elections. So much spinning, smoke, and the shade of falsehoods. Of course the Russians will interfere. As they have done in the past. As we have done in their elections in the past. As the Chinese do. And as we do in the elections of our allies. Everybody does it and as far as anyone can tell, to zero effect.

But charges of interference drive alarm and alarm drives intelligence agency budgets. Of course they will continue making these alarming claims, despite the accruing analysis and empirical evidence. Intelligence agency budgets don't just increase by themselves. You have to create perceived crises.

It is useful and important to know that interference activities are taking place but there is no point in being alarmed as long as there are no demonstrable effects of those interventions.