With Labour in the box seat to win power at the election, scrutiny focuses ever more closely on the party's economic policies.
After all, Angela Rayner, the party's deputy leader, has promised that she will bring forward a radically Left-wing employment rights bill in Labour's first 100 days in power. Sir Keir himself has publicly endorsed this so-called new deal for working people. But rather than being pro-growth, this would in fact be firmly statist and heap costs and red tape on to private firms.
Lord Mandelson, the architect of New Labour who has Sir Keir's ear, understands the importance of not deterring business and investment. This presents Sir Keir with a huge headache. Does he side with the peer and the enterprise-friendly wing of his party – or his hot-headed deputy, who will bridle at having her plans watered down?